First of all, as previously mentioned, both the new calendar and pets systems are in the game, along with that spiffy new achievements system. But there's more new stuff -- while focus is already usable on the live realms, Blizzard is implementing "a focus frame key" -- like a shift key that will let you target your focus target just for a spell's cast instead of switching back and forth all the time. And there is now an "aggro warning" in the game -- the first traces of Blizzard's official threatmeter we've known was coming for a while.
Sharing quests is easier on the PTR -- you can now share quests with party members at any distance, and even in raids, just by linking the quest into party chat. And one of the biggest small changes in the game will help out raid leaders and healers wanting to avoid the "PvPlague": players will get a timer showing when their PvP flag is set to drop. Small UI changes like this often go unnoticed in the patch notes in favor of the much more discussed class nerfs and buffs, but they go a long way towards making the game a lot more fun to play.